A light source device uses a light emitting diode (LED) for document illumination of an image reading apparatus, such as a facsimile machine or an electronic copy machine, which electrically reads and processes image information of a document, a book, or the like.
Conventionally, in an image forming apparatus, an LED array having a plurality of LED chips arranged in a line is widely used in order to obtain a linear illumination along a CIS. Recently, with high luminance of the LED chips, JP-A-9-214675 suggests a light source device for document illumination that expands light in a linear shape using a cylindrical light guide member and a pair of LED lamps arranged at both end surfaces of the light guide member.
The above-described light source device for document illumination includes an LED lamp 26a facing one incident surface 25a of a light guide member 25 and an LED lamp 26b facing the other incident surface 25b of the light guide member 25 (see FIGS. 3 and 4). In both LED lamps 26a and 26b, each turn-on period in an illumination cycle is adjusted (duty adjustment) based on the start of the illumination cycle as shown in FIG. 23. That is, turn-on timings of the LED lamps 26a, 26b in an illumination cycle are the same. Accordingly, as shown in FIG. 14 as the related art, the turn-on periods of the LED lamps 26a and 26b are aligned with each other when scanning in a sub scanning direction. Then, there is a period when the document is not illuminated while scanning a range of a feed amount for a maximum resolution in the sub scanning direction. The period of non-illumination causes lack of image data when an image reading apparatus reads the document.